BACTERIAL WILT OF CUCURBITS. 27 



and the resulting fraction reduced to a decimal. In making up the 

 relative-virulence graphs these fractions were used in the abscisses 

 to express varying degrees of relative virulence between possible 

 instant death (unity) and no infection at all (zero). For example, 

 one isolation gave (1) incipient wilt in 4 days, (2) wilt of 

 the inoculated leaf in 5 days, and (3) wilting of the whole plant 

 in 11 days after inoculation. Added together, these three figures 

 equal 20, the number used as the basis of comparison. The expres- 

 sion 1 over 20 reduced to a decimal equals 0.05, which is the figure 

 taken as the final index expressing the degree of relative virulence 

 for this particular isolation of BaciUus tracheiphilus. This happens 

 to be one of the most virulent isolations found thus far in our study. 



The relation in number of days between 1 and 2 and between 2 

 and 3 tended to be similar in the different isolations tested. That is, 

 if the first signs of wilt appeared late a longer time usually elapsed be- 

 fore complete wilting than when the first signs appeared early. How- 

 ever, this rule was not strictly true in all cases, so it was felt that a 

 truer comparison could be made by adding together the three cardinal 

 points above mentioned rather than by taking any one of them alone 

 as the basis for comparison. 



In the graphs the ordinates show the number of isolations found by 

 numerous inoculation tests to fall at any particular degree of relative 

 virulence. In plotting the curves the average virulence index for all 

 tests with any one isolation was in each case taken. 



One hundred and three different isolations were tested as to rel- 

 ative virulence in Chicago Pickling cucumber, and this list included 

 isolations from cucumber, squash, and cantaloupe. Referring to the 

 graph (fig. 6-B), it will be seen that most of the isolations are extremely 

 virulent. However, a few isolations worked very slowly, and one very 

 weak isolation never caused anything more than incipient though 

 undoubtedly true bacterial wilt. It will be noted that most of the 

 isolations (84 in number) fall above the virulence index 0.033, and it 

 may be added that all the isolations from squash are within this group. 



As 'a supplementary test, 62 of these isolations from cucumber, 

 squash, and cantaloupe were further tested as to relative virulence 

 when inoculated into White Bush Scallop squash, the method used 

 being the same as in the cucumber tests above described. Reference 

 to the graph (fig. 6-A) shows the opposite kind of a curve from that 

 obtained with cucumber as indicator. Only 12 out of the 62 isolations 

 tested caused wilting of entire squash plants, and a lower degree of 

 relative virulence was shown here than when these same isolations 

 were tested by inoculation into cucumber. Of these 12 highly viru- 

 lent isolations it happens that 6 were originally obtained from cucum- 

 ber and 6 from squash. Eighteen other isolations tested on squash 

 exhibited various shades of virulence from incipient wilt only up to 



